This invention relates generally to stabilizing an antenna mounted on a movable object, and more particularly to the stabilization of an electronically steered monopulse antenna.
In many applications it is important for an antenna on a movable object to be pointed in a particular direction, i.e., be stabilized. In one important class of systems the movable object is an aircraft and the fixed direction is measured in relation to obfects on the ground. For example, a synthetic aperture radar (SAR) on an aircraft transmits radar pulses toward a particular point on the ground and processes the reflections from an illuminated area around the particular point. Because wind or other forces may cause the aircraft to roll, pitch and yaw, the antenna of the radar must be movable relative to the aircraft to keep the centerline of the radar beam in coincidence with the particular point on the ground.
One method of stabilizing an antenna requires that the antenna be mounted on a platfor, often called "gimbals," which may be moved relative to the aircraft as required to keep the centerline of the beam fixed in space. Thus, an inertial navigation system (also called INS) in the aircraft may include gyroscopic sensors which measure the roll, pitch and yaw of the aircraft. Based on such measurements, a required movement of the gimbals to keep the beam of the antenna pointed toward a desired point on the ground is computed and the gimbals are then caused to be moved accordingly.
A major drawback of such a system for stabilizing an antenna is that gimbals which may move an antenna to compensate for roll, pitch and yaw of the aircraft must have three degrees of freedom of motion. Such gimbals are mechanically complex. A simpler gimbal with one less degree of freedom could be used with an antenna mounted so as to be insensitive to rotation around one axis. In other words, an antenna which receives the same signal regardless of how it is rotated about a particular axis does not need to be moved about that axis to compensate for motion of the aircraft about that axis.
As is known, when the beam from an array antenna is electronically steered, the antenna itself is not physically moved. Instead, the antenna elements making up an array antenna are controlled so as to change the direction of the centerline of the beam from such antenna. Thus, electronic control signals to the antenna produce the same result as physically moving the antenna on gimbals so that there is no need for mechanical motion of an array antenna to accomplish stabilization.
Planar phased array antennas have been used to stabilize antennas on aircraft because they can be electronically steered to compensate for motion of the aircraft in two directions--pitch and yaw, for example. Additionally, phased array antennas may be constructed to be insensitive to rotation in a third direction--roll, for example. In such systems, no mechanical gimbal is needed at all. The antenna may be stabilized fully by electronic steering.
However, not all planar phased array antennas are insensitive to rotation. Monopulse antennas, when formed from planar phased array antennas, are sensitive to rotation. Because monopulse antenna systems provide important advantages in many cases, planar phased array monopulse antennas are often mounted on a gimbal which moves with one degree of freedom. The antenna is thus stabilized by electronically steering the antenna in two directions and physically moving it in a third. For example, the antenna would be electronically steered to compensate for pitch and yaw of the aircraft and mounted on a gimbal with one degree of rotational freedom to compensate for roll. Gimbals with one degree of rotational freedom are simpler to construct than those with two or three degrees of freedom. However, it would be desirable to stabilize a monopulse antenna without requiring any mechanical motion.